This is an assumption based upon the comments of later writers beginning with Justin Martyr from the middle of the 2nd Century onwards.
This assumption cannot be ABSOLUTELY SUBSTANTIATED BEYOND A DOUBT!
WHY do I say this?
Because, just like the Bible, NONE OF THE ORIGINAL DOCUMENTS of the Earliest Post-Biblical writers have survived until our day.
But the converse applies to me also!
I cannot prove beyond a doubt myself that their original writings did have the Tetragrammaton ( יהוה) JEHOVAH either!
So what's my point?
What I CAN SHOW BEYOND A DOUBT is that the "NAME OF GOD", was of great importance and was indeed held in very high esteem in some of THE EARLIEST Post-Biblical Christian writers.
More specifically professed Christian writers from BEFORE the era of the Apologists 150 C.E.
"Our Common Era" (which is the equivalent of A.D. for others).I'm going to focus on three Early Christian writings:
1. Ignatius of Antioch
2. The Didache or "the Teaching of the Twelve Apostles" by an anonymous writer.
2. The First Epistle to the Corinthians by Clement of Rome.
The Epistle of Ignatius to the Trallians, Chapter VIII - "For says the prophet, as in the person of God, "Woe to him by whom my name is blasphemed among the Gentiles."
The Epistle of Ignatius to the Romans, Introduction - "Which I also salute in the name of Almighty God, and of Jesus Christ His Son..."
The Epistle of Ignatius to the Philadelphians, Chapter X - "That he may rejoice along with them when they are met together, and glorify the name of God."
The dating of the Didache, one of the earliest Post-Biblical Christian documents in existence, is shown below in the introductory note from (Roberts & Donaldsons ANF):
THE TEACHING OF THE TWELVE APOSTLES/THE LORDS TEACHING THROUGH THE TWELVE APOSTLES TO THE NATIONS - TIME AND PLACE OF COMPOSITION:
"Granting the general authenticity of the Greek work, the time of composition must be at least as early as the fist half of the second century. If the teaching is older than Barnabas, then it cannot be later than A.D. 120. If both are form a common source, the interval of time was probably not very great. The document itself bears many marks of an early date:
(1.) Its simplicity, almost amounting to childishness, not only discountenances all idea of forgery, but points to the sub-apostolic age, during which Christianity manifested this characteristic. [simple monotheism, not trinitarianism] The fact is an important one in the discussion of the canon of the New Testament.
(2.) The undeveloped [they mean trinitarian] Christian thought, as well as the indications of undeveloped heresy, confirms this position. Christianity was at first a life, for which the Apostles furnished a basis of revealed thought. But the Christians of the sub-apostolic age had not consciously assimilated the [trinitarian] thought to any large extent, while their ethical striving was stimulated by the gross sins surrounding them.
(3.) The Church polity indicated in the Teaching is less developed than that of the genuine Ignatian Epistles, and shows the existence of extraordinary traveling teachers (“Apostles” and “Prophets,” chap. xi.). This points to a date not later than the first half of the second century, probably as early as the first quarter." [italics etc added]
This is what some others have had to say about the date of the Didache:
"Hence a date for the Didache in its present form later than the second century must be considered unlikely, and a date before the end of the first century probable." (Jonathan Draper, Gospel Perspectives, v. 5, p. 269) Footnote (op. cit., p. 284), "A new consensus is emerging for a date c. 100 AD." (Jonathan Draper, Gospel Perspectives)
So there is a general opinion that the Didache, (though some disagree), that it dates from the late 1st Century to the early 2nd Century.
Just a caution and clarification here:
There are un-scriptual concepts in the Didache! These are not inspired writings!
I personally do not take the writings, and the traditions of men, the so-called "Apostolic Fathers" as important doctrinally as the Bible.
But I do want to use them to prove the point that the EARLIEST of these writers DID take "GOD'S NAME" very seriously!
Lets have a look a what they had to say about "God's NAME"! Below is several translations for comparison, which is always instructive:
Didache ( 100 c.e. ) Translated By J.B.Lightfoot.Chapter 8:2 “...but as the Lord commanded in His Gospel, ‘thus pray ye: ‘Our Father, which art in heaven,‘hallowed be Thy NAME...”
The Didache - A Translation by Rick Brannan 2009 Chapter § 8:2 “...Pray in this way: Our Father who [is] in heaven, may your NAME be holy...”
The Teaching Of The Twelve – Translated by S. Stanhope Orris P.H.D. 1884. Chapter § 8:2 “...Our Father who art in heaven, hallowed be thy NAME...”
The Teaching Of The Twelve Apostles – Translated by Roswell D Hitchcock & Francis Brown. Chapter § 8:2 “...Thus pray: Our Father who art in heaven, hallowed be thy NAME...”
THE TEACHING OF THE TWELVE copyright 1998 translated by Ivan Lewis from extant Greek manuscripts with consideration given to the Coptic and Latin text CHAPTER 8:2 "...but as commanded in the Gospel in this manner: Our Father in heaven, Sacred is Your NAME..."
THE DIDACHE (The Teaching) with comments by Ben H. Swett 30 January 1998; CHAPTER 8:2 "...but as the Lord commanded in his gospel: "Our Father in heaven, hallowed be your NAME..."
Specifically the NAME of the Father is to held "holy" or "hallowed" quoting from Matt 6:9; from the Lord's prayer in the Sermon on the Mount.
The Didache - A Translation by Rick Brannan 2009 Chapter § 10:2,3; “We give thanks to you, O Holy Father, for your holy NAME which you caused to dwell in our hearts ... 3 You, All-Powerful Master, you have created all things for the sake of your NAME...”
The Teaching Of The Twelve – Translated by S. Stanhope Orris P.H.D. 1884. Chapter § 10:2,3; “We thank Thee, Holy Father, for Thy holy NAME, which Thou hast caused to dwell in our hearts, … Thou Almighty Sovereign didst create the universe for Thy NAMES sake...”
The Teaching Of The Twelve Apostles – Translated by Roswell D Hitchcock & Francis Brown. Chapter § 10:2,3; “...We thank thee, Holy Father for thy holy NAME, which thou hast caused to dwell in our hearts … Thou Master Almighty, didst create all things for thy NAMES sake...”
Ah! Now we get specific! Yes the FATHER is the one who created the universe, ie., the "CREATOR" and not only that, he is the "ALMIGHTY SOVEREIGN".
You can check the context here, there is nothing to suggest that Jesus or the holy spirit is considered the Creator as-well.
But notice "THY HOLY NAME" is again of top priority in prayer! And a mater of thanks! This also strongly indicates the writer must have known this NAME "to dwell in" their "hearts" indicating they also had a strong love for the NAME of the Father.
Also he "created all things were for THY NAMES sake"
The Didache - A Translation by Rick Brannan 2009 Chapter § 14:3 “For this is what was said by the Lord: “In every place and time, offer me a pure sacrifice because I am a great king,” says the Lord, “and my NAME [is] great among the nations.” [Footnote 68: Mal 1:11, 14]
The Teaching Of The Twelve – Translated by S. Stanhope Orris P.H.D. 1884. Chapter § 14:3 “...For this is the one that was commanded by the Lord: 'At every place and in every place time, offer me a pure sacrifice for I am a great King, and my NAME is wondeful among he nations...”
The Teaching Of The Twelve Apostles – Translated by Roswell D Hitchcock & Francis Brown. Chapter § 14:3 “...For this is that which was spoken by the Lord: 'At every place and time, bring me a pure sacrifice for a great king am I, saith the Lord, and my NAME is marvelous among he nations...”
This is a quotation from Malachi 1:11,14; which in the ORIGINAL HEBREW has the Tetragrammaton:
Malachi 1:11 Hebrew OT: BHS (Consonants & Vowels)
כִּי מִמִּזְרַח־שֶׁמֶשׁ וְעַד־מְבֹואֹו גָּדֹול שְׁמִי בַּגֹּויִם וּבְכָל־מָקֹום מֻקְטָר מֻגָּשׁ לִשְׁמִי וּמִנְחָה טְהֹורָה כִּי־גָדֹול שְׁמִי בַּגֹּויִם אָמַר יְהוָה צְבָאֹות׃
Malachi 1:11 Greek OT: Septuagint with Diacritics
διότι ἀπ' ἀνατολῶν ἡλίου ἕως δυσμῶν τὸ ὄνομά μου δεδόξασται ἐν τοῖς ἔθνεσιν καὶ ἐν παντὶ τόπῳ θυμίαμα προσάγεται τῷ ὀνόματί μου καὶ θυσία καθαρά διότι μέγα τὸ ὄνομά μου ἐν τοῖς ἔθνεσιν λέγει κύριος παντοκράτωρ
Philippians 2:11: “And every tongue acclaim Jesus Christ as Master, ( to ) the glory of God the Father.” - THE AUTHENTIC NEW TESTAMENT by Hugh J. Schonfield, 1955.
Clement of Rome is said by some to possibly be the companion of Paul in Phillipians 4:3.
PHILLIP SCHAFFS HISTORY OF THE CHURCH Vol 2; Chap 13:162. § 162. CLEMENT OF ROME. (I.) "The Epistle of CLEMENS Rom. to the Corinthians. Only the first is genuine, the second so-called Ep. of Cl. is a homily of later date."
CLEMENT OF ROME (fl. c.90-100). "Prominent early Roman presbyter-bishop. Perhaps already mentioned in Philippians 4:3 (if from Rome) ...he is most probably the Clement in Hermas’s Shepherd whose duty was to write to other churches, which accords with the traditional authorship of 1 Clement, a letter in the name of the Roman to the Corinthian Church..."
Translated by J. B. Lightfoot - 1Clem Chapter 43:6: “What think ye, dearly beloved? Did not Moses know beforehand that this would come to pass? Assuredly he knew it. But that disorder might not arise in Israel, he did thus, to the end that the Name of the true and only God might be glorified: to whom he the glory for ever and ever. Amen...”
Translated by J. B. Lightfoot - 1Clem Chapter 45:7: “Or were Ananias and Azarias and Misael shut up in the furnace of fire by them that professed the excellent and glorious worship of the Most High? Far be this from our thoughts. Who then were they that did these things? Abominable men and full of all wickedness were stirred up to such a pitch of wrath, as to bring cruel suffering upon them that served God in a holy and blameless purpose, not knowing that the Most High is the champion and protector of them that in a pure conscience serve His excellent Name: unto whom be the glory for ever and ever. Amen.”
Translated by J. B. Lightfoot - 1Clem Chapter 47:7: “And this report hath reached not only us, but them also which differ from us, so that ye even heap blasphemies on the Name of the Lord by reason of your folly, and moreover create peril for yourselves.”
I have given the context here:
Translated by J. B. Lightfoot - 1Clem Chapter 48:1: “Let us therefore root this out quickly, and let us fall down before the Master and entreat Him with tears, that He may show Himself propitious and be reconciled unto us, and may restore us to the seemly and pure conduct which belongeth to our love of the brethren.”
Translated by J. B. Lightfoot - 1Clem Chapter 58:1: “Let us therefore be obedient unto His most holy and glorious Name, thereby escaping the threatenings which were spoken of old by the mouth of Wisdom against them which disobey, that we may dwell safely, trusting in the most holy Name of His majesty.”
Translated by J. B. Lightfoot - 1Clem Chapter 59:2: “...but we shall be guiltless of this sin. And we will ask, with instancy of prayer and supplication, that the Creator of the universe may guard intact unto the end the number that hath been numbered of His elect throughout the whole world, through His beloved Son Jesus Christ, through whom He called us from darkness to light, from ignorance to the full knowledge of the glory of His Name.”
Translated by J. B. Lightfoot - 1Clem Chapter 59:3: “[Grant unto us, Lord,] [ ἐλπίζειν ἐπὶ τὸ ἀρχεγόνον πάσης κτίσεως ὄνομά σου,] that we may set our hope on Thy Name which is the primal source of all creation, and open the eyes of our hearts, that we may know Thee, who alone abidest Highest in the lofty, Holy in the holy; who layest low in the insolence of the proud, who settest the lowly on high, and bringest the lofty low; who makest rich and makest poor; who killest and makest alive; who alone art the Benefactor of spirits and the God of all flesh; who lookest into the abysses, who scanest the works of man; the Succor of them that are in peril, the Savior of them that are in despair; The Creator and Overseer of every spirit; who multipliest the nations upon earth, and hast chosen out from all men those that love Thee through Jesus Christ, Thy beloved Son, through whom Thou didst instruct us, didst sanctify us, didst honor us. [Footnote from CCEL.org Greek text [* There appears to be a lacuna in the Greek: Lightfoot supplies Δὸς ἡμῖν, κύριε.]]
Translated by Kevin P. Edgecomb in Berkeley, California 2006; - 1st Clement Chapter 59:3: [ Δὸς ἡμῖν, κύριε ἐλπίζειν ἐπὶ τὸ ἀρχεγόνον πάσης κτίσεως ὄνομά σου,] “Grant us, Lord, to hope in the Author of all creation,Your Name opening the eyes of our heart, in order to know You alone: “resting in the Height among Heights, the Holy among Holies,” “humbling the hubris of the proud,” “the Breaker of the reasoning of the nations,” “lifting high the humble” and “humbling the high,” “making rich and making poor,” “killing and making alive,” the only Discoverer of spirits and God of all flesh, “looking into the depths,” the Observer of human works, the Helper of the endangered, the Savior of the despairing, the Creator and Overseer [episkopon] of every spirit, multiplying the nations upon the earth, choosing from all those who love You through Jesus Christ Your beloved Child, through Whom You have disciplined, sanctified, and honored us.”
Translation by Rick Brannan 2005; - 1st Clement Chapter 59:3: [ Δὸς ἡμῖν, κύριε ἐλπίζειν ἐπὶ τὸ ἀρχεγόνον πάσης κτίσεως ὄνομά σου,] “Grant us, Lord, to hope in the ultimate source of all creation, your name,...”
(ἀρχεγόνον) Variously translated as: “Arch-begotten, Primal-source, Author, First-of,” in the translations I've looked at so far.
THE FIRST EPISTLE OF CLEMENT TO THE CORINTHIANS translated by Charles H. Hoole, 1885 - Chapter 59:3: [ ἐλπίζειν ἐπὶ τὸ ἀρχεγόνον πάσης κτίσεως ὄνομά σου,] “That we may hope in thy name, which is the first of all things, open the eyes of our heart to know thee, who art alone highest among the highest, holy among the holy, who puttest down the haughtiness of the proud, who scatterest the reasonings of the Gentiles, who exaltest the humble on high, and lowerest the lofty, who makest rich and makest poor, who killest and makest to live, the only benefactor of spirits, and God of all flesh, who lookest into the abysses, who beholdest the works of men, who art the helper of those in danger, the saviour of those who have lost hope, who art the maker and bishop of every soul, who makest the nations to multiply upon earth, and out of all hast chosen those that love thee through Jesus Christ thy beloved Son, through whom thou hast taught us, hast sanctified us, hast honoured us.”
Translated by J. B. Lightfoot - 1Clem Chapter 60:4: “Give concord and peace to us and to all that dwell on the earth, as Thou gavest to our fathers, when they called on Thee in faith and truth with holiness, [that we may be saved,] while we render obedience to Thine almighty and most excellent Name, and to our rulers and governors upon the earth.”
Translated by J. B. Lightfoot - 1Clem Chapter 64:1: “Finally may the All seeing God and Master of spirits and Lord of all flesh, who chose the Lord Jesus Christ, and us through Him for a peculiar people, grant unto every soul that is called after His excellent and holy Name faith, fear, peace, patience, long-suffering, temperance, chastity and soberness, that they may be well pleasing unto His Name through our High priest and Guardian Jesus Christ, through whom unto Him be glory and majesty, might and honor, both now and for ever and ever. Amen.”
It's interesting that Clement reportedly one, if not the earliest of the so-called "Apostolic Father's", and the Didache, dating I guess – very early 2nd Century, that God's NAME or ( יהוה ) still had high importance, but from around about the middle of the 2nd Century things start to change. Plato's "nameless ONE and GOOD" philosophy had a big influence on many 2nd Century Christian writers. But as mentioned before the removal of ( יהוה ) the (τετραγράμματον) was far more influential.
Philippians 2:11: “And every tongue acclaim Jesus Christ as Master, ( to ) the glory of God the Father.” - THE AUTHENTIC NEW TESTAMENT by Hugh J. Schonfield, 1955.
Going by memory, what survives of Polycarp and Papias had little to say on God's name. Ignatius, well, what can you say, which recension (LONG Greek, MIDDLE Latin/Greek, SHORT Syriac) hasn't been tampered with! I'll have another look at these authors and what they had to say about God's name ( יהוה ) when I get time.
Barnabas and the Shepard of Hermas talk about the name of God quite a bit aswell. Barnabas (which some consider to be early-to-middle 2nd Century) in particular. If I get time I'll post what he had to say.
You could say that this is one more line of evidence to (particuarly when coupled with other evidence) show that the tetragrammaton ( יהוה ) was originally in the Christian Greek Scriptures and used by the EARLIEST CHRISTIANS.
I've been focusing and working more on a project about what the Post-Biblical Christian writers, translating their texts and what they had to say about the "Trinity", which of course is ZILCH!
IRENAEUS (c. 130-200 C.E.): "...uttered the word IAO (JEHOVAH); from which THE HOLY NAME, JEHOVAH, they say, had its origin..." — (Against Heresies, Book 1, chapter 4:1; A Library of Fathers of the Holy Catholic Church anterior to the division of east and west translated by Members of the English Church; The Five Books of S. Irenaeus Bishop of Lyons translated by the Rev. John Keble, M. A. 1872)
JUSTIN MARTYR (c. 160 C.E.): “...And your most renowned historian Diodorus, who employed thirty whole years in epitomizing the libraries, and who, as he himself wrote, ... For he spoke in these words: “Among the Jews they say that Moses ascribed his laws to THAT GOD WHO IS CALLED JEHOVAH, whether because they judged it a marvellous and quite divine conception which promised to benefit a multitude of men..." - (Justins Horatory Address to the Greeks Chapter 9. ANF R&D.)
CLEMENT OF ALEXANDRIA (c. 153-217 C.E.): “...That mystic name which is called the Tetragrammaton, by which alone they who had access to the Holy of Holies were protected, is pronounced JEHOVAH, which means, “Who is, and who shall be.”...” - Fragments: Translated by William Wilson: (Collection XI: Fragment 9).
CLEMENT OF ALEXANDRIA (c. 153-217 C.E.): “...That mystic name which is called the Tetragrammaton, by which alone they who had access to the Holy of Holies were protected, is pronounced JEHOVAH (Iehovah), which means, Who is, and who shall be..." - NICETAS, Bishop of Heraclea, Quoting CLEMENT OF ALEXANDRIA, From The Catena On The Pentateuch, Published In Latin By Francis Zephyrus, P 146.